A major determinant in the development, selection and differentiation of B lymphocytes is the B cell antigen receptor (BCR) whose antibody variable regions recognize antigen and whose signaling unit (the Igod_ he- terodimer) transmits signals into the interior of the cell. Signaling pathways activated by Igod_ cytoplasmic tails control the response of the cells to antigenic selection including tolerance induction by antigen to avoid autoimmunity and antibody responses to pathogens. BCR signaling also seems to be required for mature B cell survival. Signaling through the cytoplasmic tails of Ig(x/_ requires phosphorylation of tyrosine-based ac- tivation motifs termed ITAMs. However, there is evidence mainly from work on cell lines that other con- served targets of phosphorylation in these tails including serine and threonine residues also contribute to sig- nal transduction. The present proposal aims at defining the role of these residues in B cell development and activation in the in vivo context, using targeted mutagenesis in the mouse. We will similarly analyze the roles of the cytoplasmic tails in the maintenance of mature B cells and their interplay with B-cell activation factor- mediated survival, using systems of inducible gene targeting. Knowledge about the control of B cell survival is critical for an understanding of B cell homeostasis and therapeutic intervention in B cell-mediated auto- immunity and B cell lymphoma growth. In a final part of the proposal, we will address to which extent the differentiation of B cells into the B-1 and B-2 subsets is driven by BCR specificities which are unequally distributed between the two subsets. B-1 cells are thought to play a major role in natural immune defense and to be prone to autoantibody production, whereas B-2 cells are the major players in adaptive antibody re- sponses. We will use a genetic switch allowing the cells to switch in vivo from the expression of a B-I- typical to a B-2-typical BCR and vice versa, to determine to which extent the B-1 and B-2 phenotypes re- present distinct states of activation as opposed to being determined by distinct developmental programs. This will be complemented by an attempt to switch mature B-2 ceils to a B-1 phenotype by induced inactivation of SHP- 1 phosphatase whose inactivation in B cell progenitors leads to near-exclusive production of B- 1 cells.